1. Field of the Invention
The invention concerns a junction box for connecting a plurality of screened cables, for example screened cables for connecting industrial plants for sending or receiving data. A screened cable of this kind usually includes a plurality of insulated conductors surrounded by an electromagnetic screen in the form of a conductive braid. A junction box is used to connect a branch cable to a main cable for each item of plant to be connected to the main cable, for example.
2. Description of the Related Art
To protect the data transmitted, the screens of all the screened cables connected by a junction box must be connected together. The junction box must also incorporate a conductive enclosure forming a Faraday cage around the devices connecting the conductors transmitting the data. In the case of the FIP field bus and the WORLDFIP field bus, for example, the standards NF C46 604 and IEC 1158-2 set out physical interconnection rules firstly to preserve the matching of the data transmission lines and secondly to interconnect the cable screens correctly and to ground them.
French patent applications Nos 2 666 692 and 2 666 695 describe junction boxes conforming to these standards which enable connection of a branch cable to a main cable without interrupting data traffic on the main cable, even though it is necessary to cut the conductors of the pair of the main cable to divert the data traffic to the plant by means of a first pair and then to return data traffic to the main cable by means of a second pair. These prior art junction boxes include:
a cutter device for cutting the main cable pair; PA1 insulation displacement contacts for connecting a first section of the main cable pair to a first branch cable pair; PA1 insulation displacement contacts for connecting a second section of the main cable pair to a second branch cable pair; PA1 insulation displacement contacts for connecting a drain wire running along the screen of the main cable to a Faraday cage comprising a metal box or a metalized insulative box around all the connection devices; and PA1 three cable glands for fixing the two sections of the main cable and the branch cable to the junction box and for connecting to the Faraday cage the screens of the two sections of the main cable and the screen of the branch cable. PA1 a metal plate insulated from the conductive enclosure forming the Faraday cage and incorporating: PA1 a self-locking tie for each cable passing successively through the two holes corresponding to said cable, fitting around said cable and pressing it against the plate to attach it to the plate.
A junction box of this kind has the advantage of enabling additional plant to be connected into an existing network without interrupting data traffic for any significant time. However, this type of junction box has the following drawbacks: high cost, large size and the necessity to use special tools for operating the cutter device and the insulation displacement contacts. In some applications, where it is of no consequence if the data traffic has to be interrupted to connect additional plant, or in applications in which it is virtually certain that there will not be any need to connect additional plants in the future these junction boxes are unnecessarily costly.